Method and apparatus for simultaneous porosity and chlorinity logging



` J. HMORAN sept.15,1970

` METHOD AND APPARATUS FOR SIMULTANEOUS POROSITY AND CHLORINITY LOGGINGFiled May l0, 1967 4 Sheets-Sheet 1 4 f s s www wr ID Z/ M H U0 any@ .ACMUM Mc. W N 4 d :LV1 ,o m Wm mn n na ca w K Il C A C H 0 m H c n n wf Aw 2 n 4 R f mmm .MMM HN m MNM Pf Mm JM r A w A ,6 5 W E n w YJ M m N g.,1... z mw 4, 0 an@ wvl/JM 7.

Sept 15, 1970 J. H. MORAN 3,529,160

METHOD AND APPARATUS FOR SIMULTANDOUS POROSITY AND CHLORINITY LOGGINGFiled May 10, 1967 4 Sheets-Sheet S.

Nfl/UPON Z FL 0X Q, FROM JOU/PCE /N PHA JE 4i Z DETECTOR J/GA/AL /V/V-POHUJ fr0/PMA 770A/ 007' 0F PHAJ Sept. 15, 1970 J. H. MORAN 3,529,160

METHOD AND APPARATUS FOR SIMULTANEOUS POROSITY AND CHLORINITY LOGGING J.H. MOR'AN CHLORINITY LOGGING METHOD A-ND APPARATUS FOR SIMULTANEOUS'-,POROSIvT-Yy AND Sept. 15, 1970.

Filed May lO 1967 United States Patent O 3,529,160 METHOD AND APPARATUSFOR SIMULTANEOUS POROSITY AND CHLORINITY LOGGING .lames H. Moran,Danbury, Conn., assignor to Schlumberger Technology Corporation,Houston, Tex., a corporation of Texas Filed May 10, 1967, Ser. No.637,548 Int. Cl. G01v 5/00 U.S. Cl. Z50- 83.1 12 Claims ABSTRACT F THEDISCLOSURE iOne embodiment of the invention shows a Well logging toolthat contains a source for emitting neutron pulses of of equ'al durationand spacing. An epithermal or thermal and epithermal neutron detectorsignal registered during each pulse is added to a detector signalregistered between each pulse in order to produce an indication offormation porosity. The difference between these detector signals isdivided by the detector signal sum to provide a normalized log ofthermal neutron decay time that characterizes the chlorine concentrationin the formation. The technique also can be applied to measureepithermal or thermal and epithermal neutron slowing down time. Theneutron population, moreover, can *be measured through the gammaradiation that results from neutron interaction with the nuclearstructure of the earth formation.

This invention relates to nuclear well logging and is particularlydirected to novel methods and apparatus for nuclear well logging whichpermit simultaneous measurements of the porosity and chlorinity of theformations surrounding a borehole.

In determining information concerning the subsurface formationssurrounding boreholes, such as oil wells, two of the most importantcriteria are porosity and chlorinity. Obviously, porosity is importantsince gas, oil, or water can only be contained in porous formations.Chlorinity is important since the amount of chlorine present in a porousformation indicates whether the formation contains salt water or is alikely source of oil, gas, or fresh water. Numerous methods andapparatus have been proposed heretofore for measuring these factors.Moreover, since the development of borehole accelerators which permitirradiation of the formations with high-energy neutrons, various methodsand apparatus employing such accelerators have been proposed forobtaining information concerning the formations surrounding theborehole. However, most of the prior art techniques halve requiredaccelerators which could emit neutron bursts or pulses of extremelyshort duration, generally of the order of a few microseconds. Suchtechniques place severe requirements on the accelerators and greatlyincrease the cost, complexity, and fragility of the accelerators.Moreover, the electronic circuitry required for such techniques ishighly complex. Consequently, the cost of the apparatus necessary forsuch techniques has been high.

These disadvantages of the prior art are overcome with the presentinvention and novel methods and apparatus for nuclear well logging areprovided which permit porosity and chlorinity logs to be madesimultaneously while employing apparatus which is rugged inconstruction, simple in circuitry, and relatively inexpensive toproduce.

The advantages of the present invention are preferably attained byproviding novel methods and apparatus for nuclear Well logging whereinthe formations surrounding the borehole are irradiated with high-energyneutrons from a source which is turned ON for a period which issubstantially longer than the time required for the rate of neutronemission from source to reach an equilibrium ICC condition and which isturned OFF for an equally long period. Thermal neutrons or capture gammarays are detected during the entire cycle or during portions of the 'OFFand ON periods. However, the detector is modulated to send signals toone signal channel during the period when the source is turned ON and tosend signals to a second signal channel during the period when thesource is turned OFF. The signals in the second channel are added tothose of the iirst channel to provide a porosity log and in anotheroperation are substracted from the signals of the first channel. Thedifference is then divided by the sum to provide a log of thermalneutron decay time which indicates the chlorinity of the formations.

In another embodiment of the invention a second detector is providedwhich measures epithermal neutrons. The second detector is modulated, inthe same manner as the thermal neutron detector, to provide signals fortwo additional signal channels, and sum and difference of these signalsare normalized to provide an indication of the slowing-down time of theformations. This serves to provide and indication of the hydrogencontent in formations of low porosity.

Accordingly, it is an object of the present invention to provide novelmethods and apparatus for nuclear well logging.

Another object of the present invention is to provide novel methods andapparatus for simultaneously measuring the porosity and chlorinity offormations surrounding aborehole.

A further object of the present invention is to provide novel apparatusfor simultaneously measuring the porosity and chlorinity of formationssurrounding a borehole, which apparatus is rugged in construction,simple in circuitry, and relatively inexpensive to produce.

A specific object of the present invention is to provide novel methodsand apparatus for nuclear well logging whereby the formations suroundingthe borehole are irraditated with high-energy neutrons from a source.Thermal neutrons or capture gamma rays are detected during the entirecycle with a detector which is modulated to provide signals to a firstsignal channel while the source is turned ON and to a second signalchannel when the source is turned OFF. The signals in the two channelsare added to provide a porosity log and in a separate operation aresubtracted. The sum and difference are then normalized to provide anindication of the thermal neutron decay time which indicates thechlorinity of the formations.

Another specific object of the present invention is to provide novelmethods and apparatus for nuclear well lo gging whereby an epithermalneutron detector is employed and is modulated to provide signals for twosignal channels which may be normalized to provide an indication of theslowing-down time of the formations as an indication of the hydrogenindex information of low porosity.

These and other objects and features of the present invention will beapparent from the following detailed description taken with reference tothe figures of the accompanying drawings.

In the drawings:

FIG. 1 is a diagrammatic representation of apparatus for nuclear welllogging embodying the present invention;

FIGS. 2er-2f are diagrammatic representations of the modulation schedulefor the apparatus of FIG. 1;

FIG. 3 is a diagrammatic representation of a modified form of theapparatus of FIG. 1; and

FIG. 4 is a diagrammatic representation of a further modified form ofthe aparatus of FIG. 1.

In the forms of the present invention chosen for purposes ofillustration in the drawings, FIG. 1 shows subsurface instrument 2suspended by means of a cable 4 in a borehole 6 which penetrates theearth 8. The borehole 6 may be cased or uncased and may be empty, asshown, or may be filled with iluid, such as drilling mud, oil, water, orthe like. Cable 4 serves to raise and lower the instrument 2 in theborehole and also provides electrical connections between the instrument2 and the surface equipment. This is conventional in the well loggingart and a detailed description is believed to be unnecessary.

The subsurface instrument 2 comprises a pressure-resistant housing 10and is provided with means, such as bow spring 12, for urging one side14 of the instrument 2 into engagement with the wall of the borehole 6.Within the instrument 2 is a source 16 of high-energy neutrons. Theneutron source 16 must be capable of being turned ON and OFF so as toirradiate the formations surrounding the borehole with high-energyneutrons during alternate periods, as indicated in FIG. 2a. Thefrequency of the cycle of operation is such that the neutron source 16is turned ON for a period which is substantially longer than the timerequired for source 16 to reach an equilibrium condition and is turnedyOFF for a period of equal time. Timing cycles wherein the source 16 isturned ON for 100 microseconds or more and OFF for an equal period havebeen found satisfactory. An accelerator-type neutron source, such asthat disclosed in U.S. Pat. No. 2,991,364, issued July 4, 1961, to ClarkGoodman and assigned to the present assignee, is quite satisfactory forthe purposes of the present invention. Neutron sources of this typenormally emit 14 million-electron-volt neutrons by means of thedeuterium-tritium reaction. However, for purposes of the presentinvention, it is only necessary that the neutrons emitted by source 16have energies above the thermal energy of the formations surrounding theborehole. A thermal neutron detector 18 is mounted in the instrument 2adjacent the borehole wall engaging side 14 of the instrument 2 and isspaced a relatively long distance, of the order of inches or more, fromthe neutron source 16. Suitable shielding material 20, such as boron,paraflin, or the like, which is substantially opaque to neutrons, isprovided about the detector 18 on all sides except that adjacent side 14of the instrument 2 and substantially fills the space between thedetector 18 and neutron source 16. Suitable channels 22 and 24 areformed in the shielding material to provide conduits for electricalconnections to the source 16 and detector 18.

As indicated above, the advantages of the present invention can beobtained by detecting either thermal neutrons or capture gamma rays.Obviously, if capture gamma rays are to be detected, a gamma-raydetector would be substituted for the thermal neutron detector 18 ofFIG. 1 and appropriate shielding material, such as lead, which is opaqueto gamma rays would be disposed about the detector on all sides exceptadjacent side 14 of the instrument 2 to cause the detector to bepreferentially sensitive in the direction of side 14 of the instrument2. Such substitutions and the changes in the electronic portions of theapparatus which would be required by such substitutions are well withinthe skill of those versed in the art.

A suitable modulating device 26, such as a square-wave generator, isprovided and may be located either in the subsurface instrument 2, asshown, or in the surface equipment. The modulator 26 supplies signals toneutron source 16 to turn the source 16 ON and OFF in a predeterminedmanner, as indicated in FIG. 2a. As indicated above, source 16 operateson a time cycle whereby it is turned ON for one-half of the time cycleand is turned OFF for the other half of the time cycle. Modulator 26also provides signals to a switching circuit 28 which is connected toreceive signals from detector 18 and which serves to supply thesesignals to a first signal channel, indicated by conductor 30 during thefirst half of the timing cycle of modulator 26, and to supply thesignals to a second signal channel, indicated by conductor 32 during thesecond half of the timing cycle. Thus, conductor 30 receives thosesignals from detector 18 which are inphase with the ON period of source16 as indicated in FIGS. 2b and 2c, while conductor 32 receives thosesignals which are out-of-phase with source 16, as indicated in FIGS. 2eand 2f. Conductors 30 and 32 pass their signals to suitablesignal-processing circuits 34 where the signals are prepared forttransmission and are applied to the cable 4 to be sent to the surfaceequipment. The signal-processing circuits 34 may include amplifiers,signal-shaping circuits, blocking oscillator circuits, and otherconventional circuitry, as is well-known in the art. If desired, thesignals from detector 18 may be supplied directly to thesignal-processing circuits 34, and switching circuit 28 may be includedin the surface equipment.

With the apparatus thus far described, the neutron source 16 irradiatesthe formations surrounding the borehole with high-energy neutrons. Thetime required for most accelerator-type neutron sources to reachequilibrium, so that they emit neutrons at a constant rate, is generallyonly about a microsecond. Consequently, because of the relativelylong-time cycle employed in the method of the present invention (equalON-OFF periods of microseconds or more), this time can Safely beignored.

Most of the presently available accelerator-type neutron sources emitneutrons as a result of the deuteriumtritium reaction with energies of14 million electron volts. These neutrons gradually lose energy, due toelastic and inelastic collisions with atoms of the various elementscomposing the formations, until they reach the thermal energylevel-about l electron volt. Therefore, the neutrons emitted by thesource 16 will not reach the thermal energy level until some timeseveralmicrosecondsafter they are emitted by the source 16. Moreover, the rateat which any given neutron loses tis energy depends upon the number ofcollisions it undergoes and whether the respective collisions areelastic or inelastic. Consequently, curves, such as those of FIGS. 2band 2c, showing the counting rate supplied by detector 18 to conductor30 during the inphase portion of the cycle while source 16 is turned ON,will show a gradual increase at first, as indicated by portions 46 ofthe curves of FIGS. 2b and 2c.

As is well-known, collisions between neutrons and hydrogen nuclei willbe primarily elastic, resulting in rapid loss of energy for theneutrons. Furthermore, since the pores of all porous subsurfaceformations are filled with either oil, gas, or water, each containing alarge proportion of hydrogen, the rate at which neutrons emitted bysource 16 lose energy and arrive at the thermal energy level will bealmost instantaneous where porous formations are encountered, asindicated by portions 46 of the curves of FIG. 2c, but will be muchslower where nonporous formations are encountered, as shown by theslopes of the portions 46 of the curves of FIG. 2b. The time requiredfor the neutrons emitted by the source 16 to lose energy down to thethermal energy is referred to as the slowing-down time of the formation.Since the source 16 emits neutrons at a constant rate, the rate at whichneutrons arrive at the thermal energy level will also end to a constantvalue for any given formaion.

After the neutrons from source 16 have been slowed to the thermal energylevel, they continue to travel through the formations until they arecaptured by the nuclei of the elements composing the formations. Since,upon capture, the neutrons actually become part of the capturing atom,it will be apparent that the number of neutrons or capture gamma raysavailable for detection will depend upon the rate at which the thermalneutrons are being captured, as well as the rate at which neutrons reachthe thermal energy level. Thus, although the source 16 is emittingneutrons at a constant rate, the response of detector 18 will be afunction of the difference between the rate of arrival of neutrons atthe thermal energy level and the rate of capture of the thermal neutronsby the elements contained in the formations. In view of this, theportions 48 of the curves of FIGS. 2b and 2c will tend to become levelas the source 16 reaches equilibrium, but may actually increasedepending upon the relation between the rate of arrival of neutrons atthe thermal energy level and the rate at which the thermal neutrons arecaptured.

The likelihood that a thermal neutron will interact with a nucleus of aparticular element is proportional to the thermal neutron-scatteringcross section of that element. Although the scattering cross section ofhydrogen is only slightly greater than the cross sections of most otherelements which are likely to be encountered in the formationssurrounding a borehole, as indicated above, the pores of porousformations are filled with oil, gas, or water, all of which containlarge proportions of hydrogen. Consequently, although the neutrons fromsource 16 arrive rapidly at the thermal energy level, in porousformations the thermal neutrons will also be scattered and capturedrelatively rapidly. Therefore, the number of thermal neutrons availablefor detection by detector 18 will be relatively low, as indicated by thearea under the curves of FIG. 2c. In contrast, nonporous formationscontain relatively little hydrogen and tend to absorb thermal neutronsmore slowly. Consequently, the number of thermal neutrons available fordetection by detector 18 will be relatively high, as shown by the areaunder the curves of FIG. 2b.

After a predetermined period, modulator 26 will turn OFF source 16 sothat no additional neutrons will be emitted. At the same time, modulator26 will cause switch 28 to switch the output of detector 18 from theinphase signal channel, connected through conductor 30, to theout-of-phase signal channel, connected through conductor 32. Theout-of-phase signal from detector 18 is represented by the curves ofFIGS. 2d, 2e, and 2f. Initially, the counting rate of the out-of-phasesignal will be identical with that indicated by portion 48 of theinphase signal shown in FIGS. 2b and 2c, as seen by portions 50 of thecurves of FIGS. 2a', 2e, and 2f. This is due to the time required forneutrons emitted by source 16 shortly before source 16 was turned OFF bymodulator 26 to be slowed down to thermal energies and is comparable tothe delay represented by portion 46 of the inphase signal curves ofFIGS. 2b and'2c. Obviously, if desired, modulator 26 or switch 28 may bearranged to delay switching the output of detector 18 from inphaseconductor 30 to out-of-phase conductor 32 until this delay time haselapsed.

As indicated above, thermal neutrons continue to be scattered throughthe formations until they are captured by atoms of the elementscomposing the formations, and since during the out-of-phase portion ofthe measuring cycle no additional neutrons are being supplied by source16, it will be apparent that the number of neutrons available fordetection, during the out-of-phase portion of the cycle, will dependdirectly upon the spatial distribution of the thermal neutrons and,hence, upon the rate at which thermal neutrons are scattered andcaptured by the elements contained in the formations surrounding theborehole. This is referred to as the thermal neutron decay time of theformation and is inversely related to the scattering cross section ofthe elements in the formation. Previously, it was noted Ithat hydrogenhas a slightly larger scattering cross section than most other elementscontained in the formation and is present in large quantities in anyporous formation. Consequently, the thermal decay time for porousformations will be substantially less than that of nonporous formations.For example, the decay time for a nonporous sandstone is approximatelytwice that of a porous sandstone filled with oil or fresh water. In viewof this, the signal supplied by detector 18 to conductor 32 during the'out-of-phase portion of the measuring cycle will decline fairly rapidlyin a porous formation, as indicated by portion 52 of the curves of FIGS.2d and 2e, and will decline relatively slowly in a nonporous formation,as shown by portion 52 of the curves of FIG. 2f.

One element which will strongly affect the thermal decay time ischlorine which is usually present only in porous formations containingsalt water. Chlorine has a capture cross section for thermal neutronswhich is about times larger than the cross sections of any of the otherelements which are common in boreholes. Thus, even a slight change inthe quantity of chlorine contained in the formation will cause a largechange in the thermal neutron decay time. In a 40% porous limestonefilled with water containing 15% salt by weight, the decay time will bemuch less than if the same rock were filled with fresh Water or oil.This is illustrated by comparing the portions 52 of the curves of FIGS.2d, 2e, and 2f.

As discussed in detail above, the inphase signal, supplied by detector18 to conductor 30 when the source 16 is turned ON, will be relativelylow for porous formations, as indicated in FIG. 2c, and will berelatively high for nonporous formations, as shown in FIG. 2b. Inaddition, the out-of-phase signal, supplied by detector 18 to conductor32 when the source 16 is turned OFF, will be quite high for nonporousformations, as seen in FIG. 2f; moderate for porous formati-onscontaining oil, gas, or fresh water, as shown in FIG. 2e; and extremelylow for porous formations containing salt water, as indicated in FIG.2d. 'In view of this, it is possible, by appropriately combining theinphase and out-of-phase signals from detector 18, to providesimultaneous logs which will indicate the porosity and chlorinity of theformations surrounding the borehole.

At the surface of the earth the signals from detector 18, which aresupplied to conductor 30, during the inphase portion of the timing cyclein which source 16 is turned ON, are supplied to the inphase signalamplifier 26, as shown in FIG. l, while the signals from detector 18,which are supplied to conductor 32 during the outof-phase portion of thetiming cycle in which the source 16 is turned OFF, are supplied to theout-of-phase amplifier 38. If desired, amplifiers 36 and 38 may includeadditional components, such as counting-rate circuits and otherconventional circuits. Amplifiers 36 and 38 both supply signals toaddition circuit 40 where the signals are added to determine the totalnumber of thermal neutrons detected during the entire timing cycle. Itwill be apparent from the foregoing discussion that such a total willhave a relatively large value for nonporous formations, a relatively lowvalue for any porous formation, and an eX- tremely low value for porousformations containing salt Water. Thus, this total is indicative of theporosity of the formations and is supplied by addition circuit 40 toporosity recorder 42 which records the total as a function of depth toprovide a porosity log of the formations.

As indicated earlier, the thermal decay time of the formations isstrongly affected by the chlorinity of the formations, and where theformations are porous, the chlorinity is of interest since it providesan indication of whether the pore spaces of the formation are filledwith salt water, or are possible producing sources of oil, gas, or freshwater. Where the formations are nonporous, the chlorinity is of little,if any, interest. It has also been noted that the out-of-phase signalprovides some indication of the thermal decay time, but is alsoaffected, to some extent, by the slowing-down time required for neutronsemitted near the end of the inphase portion of the cycle to decay tothermal energies. On the other hand, it has been pointed out that inporous formations the slowing-down time is greatly reduced due to thelarge hydrogen content of porous formations. Thus, for porous formationsthe slowing-down time can be ignored, and the out-of-phase signal willprovide a rough indication of the thermal decay time, and hence, thechlorinity of the formations. Consequently, if desired, the signal fromout-of-phase amplifier 3S could be supplied directly to recorder 56 andcould be recorded as a function of depth. Where the signal on porosityrecorder 42 showed a formation to be porous, the operator could thenconsider the signal of recorder 56 to get an indication of the thermaldecay time and could thereby determine the approximate chlorinity of theformations.

tion circuit 44 to normalizing circuit 54. Normalizing circuit 54divides the signal from subtraction circuit 44 by the signal fromaddition circuit and indicates the quotient on a suitable recorder 56 asa function of depth. This quotient is a function of the thermal decaytime in porous formations and, consequently, permits lthe operator toaccurately and reliably compute the chlorinity of such formations.

With the apparatus and method thus described, it will be found that forbest results the modulating frequencies should be approximately equal tothe midpoint of the expected range of thermal decay times. Thus,frequencies of the order of 1000 cycles per second can con- -venientlybe employed for modulator 26. This means that the inphase andout-of-phase portions of the cycle may each lbe approximately 0.5millisecond in duration. Since most accelerator-type neutron sourcesreach equilibrium conditions, that is, a constant rate of neutronemission, in a period of few microseconds and since the slowing-downtime of most formations is also a matter of microseconds, modulatingfrequencies of the order of 1000 cycles per second tend to make theseeffects insignificant so that the resulting logs will not be influencedby these afctors. Moreover, modulating frequencies of this order providecounting rates which are about onehalf` or more of the counting rateswhich would be obtainable with a continuous source. In contrast, most ofthe prior art methods have employed irradiation periods of the order ofa hundred microseconds or less and have encountered serious problems inobtaining meaningful counting rates. Counting rates as low asone-twentieth of the source flux are commonly encountered with suchprior art systems, in contrast to the relatively high counting ratesobtainable with the present invention. In addition, it should be noted,as shown in FIG. 1, that the subsurface equipment required forperforming the method of the present invention is extremely simple andhence can be produced inexpensively and can be made rugged and virtuallymaintenance-free. Moreover, the method of the present invention isentirely compatible with detection systems employing pulse-heightanalysis techniques and consequently can be employed as a supplement tospectral analysis systems to provide additional information.

As indicated above, the normalization log, provided by recorder 56 ofthe apparatus of FIG. 1, 'permits an accurate determination of thechlorinity of porous formations. However, the theoretical basis for thislog requires that the slowing-down time of the formations be smallcompared to the thermal decay time. This Will be true for formations ofmoderate or high porosity due to the large hydrogen content of suchformations, as discussed above. Unfortunately, in formations of lowporosity the slowing-down time may become so large that it approachesthe thermal decay time. When this condition exists, the accuracy of thethermal decay time, as determined from the normalization log of recorder56 of FIG. l, may be questionable. Since chlorinity is normally ofinterest only as an indication of salt water in porous formations, thiscondition may be tolerable since the thermal decay time log will behighly accurate in formations of moderate or high porosity, and theporosity is indicated by the porosity log of recorder 42 of FIG. l.

Where it is necessary or desirable to know the thermal decay timeaccurately in formations of low porosity, the apparatus of FIG. 3 may beemployed. The subsurface instrument of FIG. 3 differs from that of FIG.1 only by the inclusion of a second detector 58 which is preferablyinsensitive to thermal neutrons, but detects epithermal neutrons; i.e.,neutrons having energies above the thermal energy level. Detector 58 ispositioned closer to source .16 than detector 18 and provides optimumresults when spaced 6-to-9 inches from source 16. Detector 58 is alsocontrolled by modulator 26 and switch 28 to provide an inphase signal toconductor 60 during periods when the source 16 is emitting neutrons andto provide an out-of-phase signal to conductor 62 during periods whenthe source 16 is not emitting neutrons. Since the slowing-down times ofthe formations will generally be less than microseconds, the frequencyemployed by modulator 26, in this form of the invention, will preferablybe of the order of 1000 cycles per second or more. The signals onconductors 60 and 62 are supplied to the signal-processing circuits 34and are transmitted to the surface of the earth through the cable 4, inthe same manner as described above for the signals from detector 18 ofFIG. l. At the surface, the signals from inphase conductor `60 aresupplied to an epithermal inphase amplifier circuit 64, while thesignals from out-of-phase conductor 62 are supplied to epithermalout-of-phase amplifier circuit 66. The amplifier circuits 64 and 66correspond to the amplifier circuits 36 and 38 of FIG. 1 and may includecounting-rate circuits, discriminator circuits, or other conventionalelectronic circuitry as described with respect to FIG. l. From theamplifier circuits 64 and 66, the signals are supplied to an additioncircuit 68 and a subtraction circuit 70 and these, in turn, supply sumand difference signals to normalizing circuit 72 which divides thedifference by the sum in the same manner that the thermal neutronsignals were treated in the method heretofore described with respect toFIG. 1 and supply the quotient to a suitable recorder 74 which recordsit as a function of depth.

From the normalization log of recorder 74, it is possible to compute theslowing-down time of the formations; i.e., the time during which theneutrons lost energy from the level at which they were emitted from thesource 16 to the thermal energy level. The slowing-down time, as thusdetermined, may then be used to accurately compute the thermal decaytime and chlorinity for formations of low porosity.

As a further alternative, the apparatus of FIG. 4 may be employed usinga single detector 76, such as a scintillation counter which emitselectrical signals indicative of the energy of the incident neutrons andwhich is sensitive to both thermal and epithermal neutrons. Detector 76is preferably spaced from source 16 a distance of about l2 inches and,as shown, passes signals directly to the signal-processing circuits 34for transmission through cable 4 to the surface of the earth. At thesurface, discriminator means 78 receives the signals from detector' 76and separates the signals indicati-ve of thermal neutrons from thesignals indicative of epithermal neutrons. In this instance, switch 28and modulator 26 are preferably located at the surface, anddiscriminator 78 passes the signals to switch 28 which supplies thesignals to the appropriate amplifiers 36, 38, 64 and 66 which areidentical to those of the apparatus of FIG. 3. Modulator 26 controlssource 16, through the cable 4, from the surface. The balance of thesurface equipment and the method of operation are the same as that ofFIG. 3, and modulator 26 preferably employs a frequency of about 1000cycles per second, as in the apparatus of FIG. 3.

Obviously, if desired, discriminator 78 and switch 28 could be includedin the subsurface instrument between detector 76 and thesignal-processing circuits 34. Moreover, with the apparatus of FIG. 3separate modulating and switching means could be provided-for thedetectors 18 and 58 employing respective frequencies. In addition,numerous other variations and modifications may obviously be madewithout departing from the present invention. Thus, the output of anormalizing circuit can be applied to a feedback path to control theneutron source modulator 26 in order to optimize the pulse repetitionrate and the duty cycle of the logging system. Accordingly, it should beclearly understood that the forms of the invention described above andshown in the figures of the accompanying drawings are illustrative only,and are not intended to limit the scope of the present invention.

What is claimed is:

1. A method of determining the characteristics of a substance; saidmethod comprising the steps of irradiating said substance withhigh-energy neutrons during irradiation periods separated bynonirradiation periods, the duration of said irradiation andnonirradiation periods being substantially equal and signifi-cantlylonger than the time required for the neutron source to reach`equilibrium; detecting thermal neutrons; establishing a first electricalsignal indicative of thermal neutrons detected during said irradiationperiods; establishing a second electrical signal indicative of thermalneutrons detected during said nonirradiation periods; establishing athird electrical signal indicative of the sum of said first and secondsignals; establishing a fourth electrical signal indicative of thedifference between said first and second signals; establishing a fifthelectrical signal indicative of the quotient obtained by dividing saidfourth signal by said third signal; and independently recording saidthird and fth signals.

2. A method of determining the characteristics of a substance, saidmethod comprising the steps of irradiating said substance withhigh-energy neutrons during irradiation periods of about 100microseconds separated by nonirradiation periods of equal duration withsaid irradiation periods, detecting thermal neutrons, establishing afirst electrical signal indicative of thermal neutrons detected duringsaid irradiation periods, establishing a second electrical signalindicative of thermal neutrons detected during said nonirradiationperiods, establishing a third electrical signal indicative of the sum ofsaid first and second signals, establishing a fourth electrical signalindicative of the difference between said first and second signals,establishing a fifth electrical signal indicative of the quotientobtained by dividing said fourth signal by said third signal, andindependently recording said third and fifth signals.

3. Apparatus for determining the characteristics of a substance, saidapparatus comprising means for irradiating said substance withhigh-energy neutrons during periods of irradiation separated by periodsof nonirradiation having durations substantially equal to the durationsof said periods of irradiation, means for detecting thermal neutrons,means for establishing a first electrical signal indicative of thermalneutrons detected during said periods of irradiation, means forestablishing a second electrical signal indicative of thermal neutronsdetected during said periods of nonirradiation, means for establishing athird electrical signal indicative of the sum of said first and secondsignals, means for establishng a fourth electrical signal indicative ofthe difference between said first and second signals, means forestablishing a fifth electrical signal indicative of the quotientobtained by dividing said fourth signal by said third signal, and meansfor independently recording said third and fifth signals.

4. Apparatus for determining the characteristics of a substance, saidapparatus comprising an accelerator-type neutron source capable ofirradiating said substance with high-energy neutrons, control meanscapable of turning said source ON and OFP` to obtain irradiation periodsseparated by nonirradiation periods having substantially equal durationwith said irradiation periods, a thermal neutron detector capable ofestablishing electrical signals in response to detection of thermalneutrons, first signal channel means, second signal channel means,switch means operable by said control means to pass signals from saiddetector to said first signal channel means during said irradiationperiods and to pass signals to said second signal channel means duringsaid nonirradiation periods, addition circuit means connected to receivesignals from said first and second signal channel means and capable ofestablishing an electrical signal indicative of the sum of the signalsreceived from said signal channel means, subtraction circuit meansconnected to receive signals from said first and second signal channelmeans and capable of establishing an electrical signal indicative of thedifference between the signals received from said signal channel means,normalizing circuit means connected to receive signals from saidaddition circuit means and said substraction circuit means and capableof deriving an electrical signal indicative of the quotient obtained bydividing the signal from said substraction circuit means by the signalfrom said addition circuit means, and recorder means capable ofindependently recording the signals from said addition circuit means andthe signals from said normalizing circuit means.

5. A method of determining the characteristics of a substance, saidmethod comprising the steps of irradiating said substance withhigh-energy neutrons during irradiation periods separated bynonirradiation periods having durations substantially equal to thedurations of said irradiation periods, detecting epithermal neutrons,establishing a first electrical signal indicative of epithermal neutronsdetected during said irradiation periods, establishing a secondelectrical 4signal indicative of epithermal neutrons detected duringsaid nonirradiation periods, establishing a third electrical signalindicative of the sum of said first and second signals, establishing afourth electrical signal indicative of the difference between said firstand second signals, establishing a fifth electrical signal indicative ofthe quotient obtained by dividing said fourth signal by said thirdsignal, and recording said fifth signal.

6. Apparatus for determining the characteristics of a substance, saidapparatus comprising means for irradiating said substance withhigh-energy neutrons during ir` radiation periods separated bynonirradiation periods having durations substantially equal to thedurations of said irradiation periods, means for detecting epithermalneutrons, means for establishing a first electrical signal indicative ofepithermal neutrons detected during said irradiation periods, means forestablishing a second electrical signal indicative of epithermalneutrons detected during said nonirradiation periods, means forestablishing a third electrical signal indicative of the sum of saidfirst and second signals, means for establishing a fourth electricalsignal indicative of the difference between said first and secondsignals, means for establishing a fifth electrical signal indicative ofthe quotient obtained by dividing said fourth signal by said thirdsignal, and means for recording said fifth signal.

7. A method of determining the characteristics of a substance, saidmethod comprising the steps of periodically irradiating said substancewith nuclear radiations, detecting nuclear radiations emerging from saidsubstance and resulting from said irradiation, establishing a firstelectrical signal indicative of radiations detected in time intervalsduring which said substance is being irradiated, establishing a secondelectrical signal indicative of radiations detected in time intervalsduring which said substance is not being irradiated, establishing athird electri-cal signal indicative of the sum of said first and secondsignals, establishing a fourth electrical signal indicative of l l thedifference between said first and second signals, establishing a fthelectrical signal indicative of the quotient obtained by dividing saidfourth signal by said third signal, and recording said fifth signal.

8. Apparatus for determining the characteristics of a substance, saidapparatus comprising means for periodically irradiating said substancewith nuclear radiations, means for detecting nuclear radiations emergingfrom said substance and resulting from said irradiations, means forestablishing a first electrical signal indicative of radiations detectedin time intervals during which said substance is being irradiated, meansfor establishing a second electrical signal indicative of radiationsdetected in time intervals during which substance is not beingirradiated, means for establishing a third electrical signal indicativeof the sum of said first and second signals, means for establishing afourth electrical signal indicative of the difference between said firstand second signals, means for establishing a fifth electrical signalindicative of the quotient obtained by dividing said fourth signal bysaid third signal, and means for recording said fth signal,

9. A method of determining the characteristics of a substance, saidmethod comprising the steps of irradiating said substance withhigh-energy neutrons during irradiation periods separated bynonirradiation periods of equal duration with said irradiation periods,detecting thermal and epithermal neutrons, establishing a firstelectrical signal indicative of thermal neutrons detected during saidirradiation periods, establishing a second electrical signal indicativeof thermal neutrons detected during said nonirradiation periods,establishing a third electrical signal indicative of the sum of saidfirst and second signals, recording said third signal, establishing afourth electrical signal indicative of the difference between said firstand second signals, establishing a fifth electrical signal indicative ofthe quotient obtained by dividing said fourth signal by said thirdsignal, recording said fifth signal, establishing a sixth electricalsignal indicative of epithermal neutrons detected during saidirradiation periods, establishing a seventh electrical signal indicativeof epithermal neutrons detected during said nonirradiation periods,establishing an eighth electrical signal indicative of the sum of saidsixth and seventh signals, establishing a ninth electrical signalindicative of the difference between said sixth and seventh signals,establishing a tenth electrical signal indicative of the quotientobtained by dividing said ninth signal by said eighth signal, andrecording said tenth signal.

10. A method of determining the characteristics of a substance; saidmethod comprising the steps of irradiating said substance withhigh-energy neutrons during irradiation periods separated bynonirradiation periods; detecting thermal neutrons; establishing a rstelectrical signal indicative of thermal neutrons detected during said l2irradiation periods; establishing a second electrical signal indicativeof thermal neutrons detected during said nonirradiation periods;establishing a third electrical signal indicative of the sum of said rstand second signals; establishing a fourth electrical signal indicativeof the difference between said rst and second signals; establishing afifth electrical signal indicative of the quotient obtained by dividingsaid fourth signal by said third signal; and independently recordingsaid third and fifth signals.

11. A method of determining the characteristics of a substance; saidmethod comprising the steps of irradiating said substance withhigh-energy neutrons during irradiation periods separated bynonirradiation periods, the duration of said irradiation periods beingsubstantially equal and significantly longer than the time required forthe neutron source to reach equilibrium; detecting thermal neutrons;establishing a first electrical signal indicative of thermal neutronsdetected during said irradiation periods; establishing a secondelectrical signal indicative of thermal neutrons detected during saidnonirradiation periods; establishing a third electrical signalindicative of the sum of said iirst and second signals; establishing afourth electrical signal indicative of the dilerence between said lirstand second signals; establishing a fifth electrical signal indicative ofthe quotient obtained by dividing said fourth signal by said thirdsignal; and independently recording said third and fifth signals.

12. Apparatus for determining the characteristics of a substance, saidapparatus comprising means for irradiating said substance withhigh-energy neutrons during periods of irradiation separated by periodsof nonirradiation, means for detecting thermal neutrons, means forestablishing a first electrical signal indicative of thermal neutronsdetected during said periods of irradiation, means for establishing asecond electrical signal indicative of thermal neutrons detected duringsaid periods of nonirradiation, means for establishing a thirdelectrical signal indicative of the sum of said first and secondsignals, means for establishing a fourth electrical signal indicative ofthe dilerence between said first and second signals, means forestablishing a tifth electrical signal indicative of the quotientobtained by dividing said fourth signal by said third signal, and meansfor independently recording said third and fifth signals.

References Cited UNITED STATES PATENTS 3,420,998 1/1969 Mills 250-83.1 X

ARCHIE R. BORCHELT, Primary Examiner U.S. Cl. X.R.

